Use Member self delete plugin
To delete them directly click on the little triangle on the avatar to open up the menu - and you'll have an option to delete them. Alternately if you go to their profile page below the picture will be a menu with a section called "admin options" - same thing there.
One word of caution - deleting a user without Matt's member self-delete plugin (with options set not to remove content) will delete everything they created - groups, admin settings, whatever. It's the nuclear option.
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